Sintering and Contact Formation of Glass Containing Silver Pastes

Abstract

The silver paste composition for the front side has a strong influence on the firing behaviour, contact formation and resulting efficiency of the solar cell. It is state of the art that the paste glass is needed for etching the ARC silicon nitride during firing and also plays a major role for the transport of silver to the silicon surface. However, the kinetics of the silver transport is still under discussion. A main challenge to understand the underlying mechanisms is the analysis of the microstructure evolution of the paste during firing in the high speed PV firing cycle. In this work PV high speed firing conditions were simulated by usage of a simple laboratory furnace. The microstructural evolution of a model paste was observed and correlated to obtained contact resistances. Moreover, systematic variations of the paste glass were investigated. The overall contact formation kinetics can be divided into two independent but strongly interacting single kinetic phenomena: (i) the reaction kinetics at the wafer surface and (ii) the phase transport kinetics of the silver paste. The reaction kinetic at the wafer surface is composed by three important single reactions: silver solution and precipitation in the amorphous interface layer, interface layer growth, and silicon etching. The phase transport kinetics of the metallization paste determines the quantity of glass that is mixed with the etching products in the interface layer and is dominated by the densification rate of the silver powder and the viscous flow of the paste glass. The paste glass has crucial influence for both kinetics (i) and (ii). In this study the paste glass chemistry has a dominating impact compared to the glass rheology. By an appropriate choice of the raw materials regarding the sinter behaviour and targeted selection of the powder properties substantial control over the paste contacting performance is possible.